Insulating materials made of cellulose materials, such as kraft paper, pressboard, and the like are typically used in oil-filled power transformers as electrical insulation to separate different circuits within a transformer, to provide mechanical support for the coils, to isolate the winding core and outer case from the circuits, and so forth. In order to perform these functions, insulating materials must have high dielectric strength and high mechanical strength. However, these insulating materials are gradually degraded and lose mechanical strength due to heating (pyrolysis), oxygen (oxidation), and moisture (hydrolysis), which can adversely affect the life of a transformer. For example, the loss of mechanical strength due to degradation of the insulating material causes the transformer winding to become more susceptible to mechanical damage during movement, particularly during extreme events, such as through-faults. Accordingly, the life of a transformer is limited to the life of its solid insulating material.
A technique used to measure the ability of the cellulose insulating material to withstand the stresses within a transformer is through measurement of the degree of polymerization (DP). The cellulose in insulating material is a long chain polymer with repeating glucose bonds, and a measure of the number of these repeating glucose bonds per unit is the degree of polymerization. A typical insulating material has an average degree of polymerization of approximately one thousand when new. However, as glucose bonds in insulating material are broken, the degree of polymerization is reduced. Thus, aged, i.e., degraded, insulating material with a degree of polymerization of less than two hundred may be too brittle to effectively withstand mechanical stresses within the transformer. Consequently, a test to measure the degree of polymerization can be performed to assess insulation aging.
In operating transformers, in order to directly measure the degree of polymerization, the transformer is deenergized, and samples of the cellulose insulating material are taken to a laboratory for testing. Obviously, this direct measurement methodology is time consuming and expensive. An indirect way to measure the degree of polymerization is through, furan testing. Furans are major cellulose degradation products that can be found in the transformer insulating oil. Oil samples can be analyzed for furans and compared with furan models to obtain an approximation of the degree of polymerization of the insulating material. However, this approximation technique does not always give consistent results.